Researchers create anti-fogging, self-cleaning glass

It uses a process involving thin layers of material deposited on a surface which is then selectively etched away. The team has produced a surface covered with tiny cones, each five times taller than their base width of 200nm. When used on glass, this pattern prevents reflections, while at the same time repelling water from the surface. One of the most instantly recognisable features of glass is the way it reflects light. But this way of creating surface textures on glass virtually eliminates reflections, producing glass that is almost unrecognisable because of its absence of glare. It also causes water droplets to bounce off, like tiny rubber balls. The glass is based on surface nanotextures that produce an array of conical features resulting in it being self-cleaning, resistant fogging and virtually no glare. Ultimately, the researchers hope it can be made using an inexpensive manufacturing process that could be applied to optical devices, the screens of smartphones, televisions, solar panels, car windshields and even windows on buildings. Car windows, for example, could also benefit cleaning themselves of dirt and grit on the exterior surface of the windows, eliminating glare and reflections that can impair visibility and preventing fogging on the interior surface. Additionally, Photovoltaic panels could benefit. PV panels can lose as much as 40% of their efficiency within six months as dust and dirt accumulate on the surfaces. A self-cleaning glass would have much less of a problem. In addition, the new glass would be more efficient because more light would be transmitted through its surface, instead of being reflected away, especially when the sun's rays are inclined at a sharp angle to the panel. At such times, such as early mornings and late afternoons, conventional glass might reflect away more than 50% of the light, whereas an anti-reflection surface would reduce the reflection to a negligible level. Fabrication begins by coating a glass surface with several thin layers, including a photoresist layer, which is then illuminated with a grid pattern and etched away; successive etchings produce the conical shapes. The new fabrication approach the MIT team developed using coating and etching techniques adapted from the semiconductor industry. Since it is the shape of the nanotextured surface, rather than any particular method of achieving that shape, that provides the unique characteristics, in the future glass or transparent polymer films might be manufactured with such surface features simply by passing them through a pair of textured rollers while still partially molten; such a process would add minimally to the cost of manufacture. The researchers say they drew their inspiration from nature, where textured surfaces ranging from lotus leaves to desert-beetle carapaces and moth eyes have developed in ways that often fulfil multiple purposes at once. Although the arrays of pointed nanocones on the surface appear fragile when viewed microscopically, the researchers say their calculations show they should be resistant to a wide range of forces, ranging from impact by raindrops in a strong downpour or wind-driven pollen and grit to direct poking with a finger. Further testing will be needed to demonstrate how well the nanotextured surfaces hold up over time in practical applications. The technology is described in a paper published in the journal ACS Nano, co-authored by mechanical engineering graduate students Kyoo-Chul Park and Hyungryul Choi